What is the ruling on an employee engaging in direct or indirect commercial activity after signing a written declaration not to do so?

So long as the party with whom you contracted stipulated that you should not engage in direct or indirect commercial work, and you agreed to that, then you are not permitted to violate it and practice trade without their knowledge and consent. This is based on the Almighty's saying: "O you who have believed, fulfill [your] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their stipulations."